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Optical Networks: A Practical Perspective - Part 22

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Optical Networks: A Practical Perspective - Part 22. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 180 Components Figure 3.51 Structure of a tunable micro-electro-mechanical vertical cavity surfaceemitting laser MEM-VCSEL from Vak99 . wavelength can be changed. This is a slow method of tuning since the tilt and position of the diffraction grating have to be changed by mechanical means. However a very wide tuning range of about 100 nm can be obtained for semiconductor lasers by this method. This method of tuning is appropriate for test instruments but not for a compact light source for communication systems. Tunable VCSELs We studied VCSELs in Section 3.5.1. There we saw that the main challenges in realizing long-wavelength 1.55 zm VCSELs were in obtaining sufficient cavity gain obtaining highly reflective mirror surfaces dealing with the heat dissipation and making the laser operate in a single-longitudinal mode. Figure 3.51 shows a VCSEL design Vak99 that attempts to solve these problems while also making the laser itself tunable. The tunability is achieved by having the upper mirror be a movable micro-electro-mechanical MEM membrane. The cavity spacing can be adjusted by moving the upper mirror by applying a voltage across the upper and lower mirrors. The upper mirror is curved to prevent beam walk-off in the cavity leading to better stability of the lasing mode. To conduct the heat away from the bottom mirror a hole is etched in the InP substrate. The design uses a 980 nm pump laser to pump the VCSEL cavity. Any pump wavelength lower than the desired lasing wavelength can be used to excite the semiconductor electrons to the conduction band. For example the 980 nm semiconductor pumps used to pump erbium-doped fiber amplifiers can be used here as well. By designing the pump spot size to match the size of the fundamental lasing mode the laser can be made single mode while suppressing the higher-order Fabry-Perot cavity modes. Using gain to perform this function is better than trying to design the cavity to provide higher loss at the higher-order modes. The high